1. Field of the Invention
The invention relates to an overvoltage protection element with a housing, terminals for electrical connection of the overvoltage protection element to a current path or a signal path to be protected, two varistors which are connected electrically in parallel and which are located within the housing, and a middle electrode located at least partially between the varistors. The housing has two metal housing halves which are electrically connected to one another. The middle electrode is insulated relative to the housing halves and electrically connected with its opposite sides to a first connecting region of a varistor. The two varistors and the middle electrode are sandwiched between the two housing halves.
2. Description of Related Art
Conventional electrical circuits and systems work without perturbations in the voltage specified for them (i.e., the rated voltage). This does not apply when overvoltages occur. Overvoltages are any voltages which are above the upper tolerance limit of the rated voltage. They mainly include transient overvoltages, which can occur due to atmospheric discharges, but also may occur due to switching actions or short circuits in power supply networks, which can be coupled galvanically, inductively or capacitively into electrical circuits. In order to protect electrical or electronic circuits, especially for electronic measurement, control and switching circuits, in which they may be used, against transient overvoltages, overvoltage protection elements were developed and have been used for years.
Due to aging and temporarily occurring overvoltages (TOV) in the range of seconds, an unwanted increase of the leakage current of the varistor at operating voltages occurs, especially in overvoltage protection elements with a varistor as the arrester. Overvoltage protection elements with a varistor as the arrester, therefore, nowadays often have a thermal disconnector by which the varistor, which is no longer properly serviceable, is electrically separated from the current path to be monitored. In the conventional overvoltage protection elements, the state of the varistor is monitored according to the principle of a temperature switch. When the varistor overheats, for example due to a leakage current which has occurred, a solder connection provided between the varistor and a disconnection means is broken, which leads to electrical disconnection of the varistor.
This overvoltage protection element is known, for example, from German Patent DE 695 03 743 T2. In the known overvoltage protection element, which has two varistors, that are located parallel to one another. The thermal disconnector is additionally connected to an optical state display so that the state of the overvoltage protection element can be read directly on site using the optical state display. For the optical state display, this overvoltage protection element has a first slide which is located in the housing and which is actuated by separating tongues which form the separating means, and interacts with a second slide which can be moved relative to a viewing port depending on the position of the first slide.
The disadvantage in the known overvoltage protection devices and overvoltage protection elements is, however, that each contact opening under operating voltages of greater than 30 volts and high current loads can produce an arc. Thus, when the solder connection is broken an arc can occur between the varistor and the separation means, which can lead to damage of components within the overvoltage protection element or of the overvoltage protection element overall, and especially of the plastic housing surrounding the varistor. Since several of these overvoltage protection elements or overvoltage protection devices are often arranged several next to one another and to other electronic devices, adjacent overvoltage protection devices or other electronic devices can often by destroyed or damaged due to an arc which occurs within the housing.
German Patent DE 601 12 410 T2 discloses an overvoltage protection device which has a varistor wafer which is located in a pot-shaped metal housing and which is braced against the bottom of the pot-shaped housing using a piston-shaped electrode. The housing is sealed with a cover which is either screwed into the pot-shaped housing or is attached by a spring ring or a clip which locks in a groove in the side wall of the housing. There is an opening in the cover through which the shaft of the electrode is routed out of the housing for electrical connection of the electrode. A second terminal for electrical connection of the overvoltage protection element to the current paths or signal paths to be protected is made on the housing. For electrical insulation of the electrode relative to the housing there is an insulating ring which is located within the housing and which likewise has an opening for the shaft of the electrode.
According to another version, the overvoltage protection device known from German Patent DE 601 12 410 T2 has two varistor wafers which are each braced against a middle wall of the cylindrical housing using a piston-shaped electrode. To connect the housing, a housing electrode clip is made on the housing. By making the housing out of aluminum, destruction of the housing when an arc occurs on the varistor is prevented. The electrical contact-making of the varistor and its arrangement in the housing and the arrangement and configuration of the electrodes make the structure and the mounting of the known overvoltage protection device, however, relatively complex.
German Patent DE 10 2007 030 653 A1, which corresponds to U.S. Pat. No. 8,089,745 B2, discloses an initially described overvoltage protection element which likewise has a metal housing consisting of two half shells. Both in the overvoltage protection element known from German Patent DE 10 2007 030 653 A1, which corresponds to U.S. Pat. No. 8,089,745 B2, and also in the one known from German Patent DE 601 12 410 T2, the problem is that the varistor wafers located within their housing are subject to tolerances, especially in their thickness so that to ensure a reliable contact-making of the connecting regions of the varistors for the two overvoltage protection elements, spring elements in the housing are used.
In the overvoltage protection element known from German Patent DE 10 2007 030 653 A1, which corresponds to U.S. Pat. No. 8,089,745 B2, according to the preferred embodiment there are two elastic contact elements at a time between one half shell of the housing and the assigned first connecting regions of a varistor. The electrical connection between the two housing half-shells and the two varistors can be implemented by a purely mechanical connection so that welding or soldering processes in the mounting of the overvoltage protection element are not necessary. Depending on the actual thickness of the two varistors, however, different contact forces arise and, as a result, adversely affect the reliable and continuous operation of the overvoltage protection element under certain circumstances. Moreover the use of elastic contact elements has a disadvantage due to the impedance of the elastic contact elements, which increases the impedance of the electrical connection to the varistors, and thus also raises the possible noise level. Moreover, the contact-making surface of the varistors with the housing half-shells is reduced, which further increases the impedance of the electrical connection.